When a positive voltage is applied from a facing electrode to a metal processed to be in a needle shape, an electric field is concentrated on a front end of the metal. When a strength of the electric field is about 107 V/cm, free electrons in the metal may escape a potential barrier of the surface due to the tunneling effect and be externally emitted. This phenomenon is referred to as a field emission.
An apparatus for acquiring electron beams by the field emission is referred to as a field emission electron gun (FEG). As for an electron source of the FEG, a tungsten beam formed of a single crystal that is sharpened to be in a needle shape is generally used and is used at room temperature. The electron source and an extraction electrode facing the electron source are installed in a vacuum container and an electron is emitted by applying an extraction voltage to the electron source and the extraction electrode. The emitted electron is accelerated at a high pressure applied to an acceleration electrode to form an electron beam.
An element of determining a performance of the FEG includes a brightness and an energy spread. The brightness indicates an amount of electron beam as a value indicating how much an electron beam of current can be obtained per unit solid angle, from the electron source per unit area. The energy spread indicates a monochromaticity of an electron beam as a range of wavelengths of the electron beam. Since the FEG can obtain an electron beam having a high brightness and a narrow energy spread compared to other thermionic emission type or Schottky emission electron gun, the FEG is used, as a high resolving power electron gun, for the charged particle beam apparatus such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
As a problem of the FEG, it is known that emission current is unstable. FIG. 1 illustrates a representative time change of current. The same time change is disclosed in patent document 1. Current emitted from the clean surface of an electron source significantly decreases immediately after the emission and continues its gradual decrease even after the significant decrease. Next, noise starts occurring in the current and increases over time. Then, the current changes to increase and noise further increases. A period where a decrease in current is significant, which is indicated by A in FIG. 1, is referred to as a decreasing region, and a period where the decrease in current is gradual, which is indicated by B in FIG. 1, is referred to as a stable region.
The time change in current of the FEG occurs by adsorbing gas residual in the vacuum on the surface of the electron source. When gas particles are adsorbed on the clean metal surface, a work function of the surface increases. As a result, the potential barrier of the surface is enlarged and a number of electrons to be emitted decreases. Since the current of electron beam decreases, a brightness is deteriorated. Also, since an extraction voltage required to obtain the same current increases, an electron having a relatively wide energy spread escapes the potential barrier whereby the energy spread of the electron beam becomes widened. When a predetermined amount of gas particles are adsorbed on the surface, a gas adsorbed layer is formed and thus, a change of the work function decreases and the current becomes relatively stable. A region after the gas adsorbed layer is formed corresponds to the stable region. Even in the stable region, gas is deposited on the adsorbed layer or the adsorbed gas of the surface is substituted with another gas whereby the current gradually keeps decreasing. The gas particles adsorbed on the surface performs desorption, substitution, or migration for a short period of time, which causes noise in current. Also, positive ions generated by the electron beam collide with the electron source, which damages the surface, causing the shape to be uneven, which is also regarded as another cause of noise.
The clean metal surface may be obtained again by performing a flashing operation of heating the electron source for a short period of time. By heating the electron source at a high temperature, the adsorbed gas of the surface is desorbed and metal atoms of the surface migrate, whereby the surface becomes smooth. Through this, the clean surface can be obtained. As a larger amount of gas is adsorbed on the surface, a higher temperature of flashing is required to clean the surface. In the meantime, the surface of the electron source is melted according to the high temperature of flashing and thus, a radius of curvature of a front end increases. When the radius of curvature increases, a strength of an electric field to be applied to the surface decreases and thus, an extraction voltage required for the field emission increases. Substantially, there is an upper limit in an extraction voltage to be applicable and an un-sharpness of diameter of the front end of the electron source by flashing determines a usage lifespan of the electron source.
When a user uses the FEG, the user operates an apparatus based on the time change of current as shown in FIG. 2. The user initially performs flashing 1 of the electron source and then performs increasing 2 of the extraction voltage to thereby emit an electron beam. Since the current significantly decreases in the decreasing region immediately after the emission, the user avoids a use of this region and uses the stable region after waiting for tens of minutes until the current enters a stable region where the decrease in current is gradual. Since the current slowly decreases even during the period of the stable region, the user maintains the current to be greater than a predetermined value by repeating increasing 2 of the extraction voltage. Every time the extraction voltage is increased, a criterion of an electron optical system changes and thus, the user may need to re-coordinate an optical axis. Also, when an observation continues for several hours, noise starts occurring in the current. The noise disturbs the usage of the FEG. The noise is eliminated by re-flashing the electron source and thereby cleaning the surface. In the case of flashing, the time change of current is returned to an initial state and thus, the user resumes the use after waiting until the current enters the stable region.
In general, when performing flashing, the user performs stop 3 of the extraction voltage and stops emitting of the electron beam once. This is because when flashing is performed in a state where the extraction voltage is applied, a protrusion of an atom level is formed on the front end of the electron source. The phenomenon where the protrusion is formed is referred to as buildup. The buildup occurs when metal atoms of the surface melted at a high temperature in flashing are drawn towards the front end by the electric field and deposited thereon. Due to the protrusion, a strength of the electric field concentrated on the front end increases and emission current increases. However, due to the protrusion, the adsorbed gas or damage effect is serious and the current easily becomes unstable. Due to the above reasons, the buildup is avoided for practical use.
As described above, even though the FEG has a high resolving power compared to other electron guns, it is inconvenient to use the FEG in that the time change of current occurs. Also, the time change of current becomes an issue with respect to an apparatus requiring a long period of stable current such as an analysis SEM, a length measurement SEM. Accordingly, it is difficult to apply the FEG. Currently, in many cases, Schottky emission electron gun having a relatively low resolving power, however, having stable current is installed in the above apparatuses.
As a method of automatically keeping maintaining the current of FEG to be greater than a predetermined magnitude, a method of intermittently performing flashing in the stable region, disclosed in patent document 1, or a method of intermittently performing flashing in the decreasing region, disclosed in patent document 2, are known. In the meantime, as a method of extending the time change of current to thereby reduce a decreasing speed, a method of enhancing a vacuum degree around the electron source using a titan sublimation pump and liquid nitrogen cooling is disclosed in non-patent document 1. Also, as an electron gun structure of enhancing the vacuum degree, a charged particle beam apparatus using a non-evaporative getter (NEG) pump is disclosed in patent document 3, and a charged particle beam apparatus including an NEG pump and an ion pump is disclosed in patent document 4. In addition, as a structure of supplying gas to an electron gun, patent document 5 is proposed.